1. Field of the Invention
The present invention relates to a capacitive sensor, and more particularly, to a capacitive sensor with alternating current power immunity.
2. Description of the Related Art
In recent years, due to the development of technology, control buttons, such as buttons of an elevator or a game console, evolve from a mechanical type of button into a touch sensor. FIG. 1 illustrates a circuit diagram depicting a capacitive touch sensor in the prior art. Referring to FIG. 1, the capacitive touch sensor includes a sensing electrode 101, a resistor 102 and a sensing-control terminal 103, wherein the sensing electrode 101 in the circuit is equivalent to a grounded capacitor Cx.
FIG. 2 illustrates an operational waveform diagram of a node A coupled to the sensing electrode 101 and the resistor 102. Referring to FIGS. 1 and 2, the sensing-control terminal 103 charges the node A to a first preset voltage V20 at the beginning, and then the node A is set to high-impedance. Afterward, since the sensing electrode 101 is equivalent to the grounding capacitor Cx, the sensing electrode 101 starts to discharge through the resistor 102. The sensing-control terminal 103 continuously detects a voltage of node A. When the voltage of node A discharges to a second preset voltage V21, the sensing-control terminal 103 determines whether a finger touches the sensing electrode according to a period when the voltage of the node A is discharged from the first preset voltage V20 to the second preset voltage, and then the sensing-control terminal 103 begins to charge the node A.
Referring to FIG. 2, the waveform 201 is a voltage waveform of node A when the finger does not touch the sensing electrode 101, and the waveform 202 is a voltage waveform of node A when the finger touches the sensing electrode 101. According to the waveforms, when the finger touches the sensing electrode 101, the equivalent capacitor of the sensing electrode 101 is increased, so that a discharge time T2 of the waveform 202 is longer than a discharge time T1 of the waveform 201. Therefore, as long as it is determined that the period when the voltage of node A is discharged from the first preset voltage V20 to the second preset voltage V21 is longer than the discharge time T1 by the sensing-control terminal, it can be determined that the sensing electrode 101 is touched.
However, the conventional capacitive sensor is much subjected to the interference of the alternating current (AC) signal, especially the interference of the AC power source in our daily life. Since the sensing electrode of the conventional capacitive sensor discharges to ground, the conventional capacitive sensor is comparative to ground to measure the equivalent capacitance of the sensing electrode. If the capacitive sensor is disposed to mobile apparatus, the ground of the sensing electrode is the battery ground. The battery ground is different from the environment's ground. When the interference of the AC power source is induced, the relationship between the battery ground and the environment ground will start to float. In other words, the outside environment observed by the battery system is a floating environment when the interference of the AC power source is existed. Thus, the measurement of the equivalent capacitance will be inaccurate.